The present invention relates generally to a coating for transparent substrates with high UV absorbency and excellent weatherability, and more particularly to a doped metal oxide coating for transparent polymeric substrates.
Car manufacturers have traditionally used glass as the material for car windows. However, recently it has become desirable to substitute glass windows with polymeric windows, such as polycarbonate (PC) windows. Polymeric windows are often advantageous compared to glass windows because of their low weight, high strength and ease of shaping. However, some polymeric windows, such as PC windows, turn yellow after being exposed to the ultraviolet (UV) radiation in sunlight. The yellow windows suffer from poor transmission of visible light, thus making them unsuitable for many car window applications.
One solution to the UV damage problem is to coat the polymeric substrates with a TiO2 (titanium dioxide) layer. The TiO2 coating acts as a partial absorber of UV radiation and provides a measure of protection for some applications. However, TiO2 generally provides an insufficient amount of UV absorption, and windows coated with TiO2 eventually turn a shade of yellow. TiO2 is also a photocatalyst for oxidation of polymers. Therefore, TiO2 coated polymers are generally unsuitable as a car window material.
Another solution to the UV damage problem is to coat the polymers with a ZnO (zinc oxide) layer. Zinc oxide provides UV absorption to higher wavelengths than TiO2, providing greater protection for polycarbonate and other polymeric substrates. Therefore, polymers coated with ZnO generally do not turn yellow when exposed to sunlight. However, prior art ZnO coatings are known to dissolve in water. Poor water soak stability on many substrates, including polycarbonate (PC), has thus surfaced as one potential limitation to the use of ZnO as a UV absorber for car windows, because car windows are frequently exposed to rain and snow.
In view of the foregoing, it would be beneficial to have a coating for a transparent substrate that exhibits good UV absorption and chemical stability. It would also be desirable to have a method for effectively applying such a coating.
According to an exemplary embodiment, the invention provides a structure which includes a polymeric substrate and a weather-resistant and UV absorbent doped zinc oxide layer on the polymeric substrate. The doped zinc oxide layer may comprise, among other examples, indium doped zinc oxide (IZO) or aluminum doped zinc oxide (AZO).
According to another aspect, the present invention comprises a window for use in a vehicle, a building, a display device, or an apparatus comprising a polymeric base and a weather-resistant UV absorption layer comprising a doped zinc oxide layer on the polymeric base.
According to another embodiment, the invention comprises a method of protecting polymeric substrates from UV degradation comprising coating the polymeric substrate with a layer comprising a doped zinc oxide layer.
The invention also relates to a method of forming an indium doped zinc oxide layer on a substrate comprising the steps of placing the substrate in a sputtering chamber, providing indium and zinc in at least one target, and performing DC magnetron sputtering on the target to form an indium doped zinc oxide layer on the substrate.